Optical sensing techniques are used to determine the position, dimensions, attitude and angular displacement of a moving object. They are also used to track and/or sense the motion of a mechanical part belonging to a larger system or perform other position measurements requiring high levels of accuracy. Among other, these techniques find-numerous applications in the fields of robotics, artificial vision, mechanical control and feedback. For an example of a prior art method and apparatus for electro-optically determining the dimension, location and attitude of objects the reader is referred to U.S. Pat. No. 6,211,506 to Pryor et al.
Most of the sensing techniques use a coherent radiation source to generate a beam of electromagnetic radiation in a wavelength range suitable for the particular environment and application. For example, the source can be a laser delivering a beam of light in the visible wavelength range. The light beam is reflected from the object whose position is to be sensed to a position sensitive photodetector (PSD). The reflected light impinges on the PSD and produces a spot whose two-dimensional extent is analyzed to find the centroid.
Locating the centroid of a light spot presents a number of challenges due to ever-increasing requirements for high sensitivity, high resolution, linearity of response, immunity to stray light and speed. PSDs are generally divided into two groups: continuous response position sensitive detectors (CRPSD) and discrete response position sensitive detectors (DRPSD). A CRPSD is a detector that determines/calculates the centroid of a light distribution that may include stray light components in addition to a desired light spot. A DRPSD is a detector that samples and analyzes the entire light distribution to determine the position of the desired light spot within the light distribution.
CRPSDs typically use lateral effect photodiodes (LEPS) and geometrically shaped photo-diodes (wedges or segmented) such as described by A. Makynen and J. Kostamovaara, “Linear and sensitive CMOS position-sensitive photodetector”, Electronics Letters, Vol. 34, No. 12, pp. 1255-56 (11 Jun., 1998) and in A. Makynen et al., “High accuracy CMOS position-sensitive photodetector (PSD)”, Electronics Letters, Vol. 33, No. 2, pp. 128-130 (16 Jan., 1997). The first of these references takes note of the nonlinearity and high noise suffered by LEPs in practical applications despite their large-area continuous construction and proposes a CMOS-compatible PDS using phototransistors to achieve higher resolution, accuracy and linearity. The phototransistors are small-sized and arranged to form a dimensionally accurate array, in which the emitters of every other phototransistors in each row are connected to the row current line and every other to the column current line. The photocurrents are processed using two separate arrays of polysilicon resistors with homogenous resistivity. The use of such array of phototransistors improves resolution in comparison to a conventional LEP and achieves good linearity. The second of these references describes further improvements to a CMOS PSD having a similar array construction to render it optimal for outdoor use.
A further review and teaching of multi-pixel PSDs using CMOS technology is found in Davies W. DeLimaMonteiro, et al., “Various Layouts of Analog CMOS Optical Position-Sensitive Detectors”, SPIE Conference on Electronics for Solid State Sensors, SPIE Vol. 3794, pp. 134 (July 1999). This reference teaches the use of CMOS technologies to produce several array geometries and interconnections including an array of photodiodes in a chessboard-like structure.
DRPSD are generally implemented using an array of photosensors that are read out serially by metal oxide semiconductor field effect transistor (MOSFET) switches or a charge coupled device (CCD) as disclosed by F. Blais and M. Rioux, “Real-Time Numerical Peak Detector”, Signal Processing, Vol. 11, No. 2, pp. 145-155 (1986). Since a DRPS samples the entire distribution, it can typically achieve higher accuracy levels than CRPSD, but at a slower speed relative to a CRPSD.
U.S. Pat. No. 6,297,488 to Beraldin et al. teaches a position sensitive light spot detector developed to improve the resolution and speed of a PSD. This detector includes a CRPSD (e.g., a lateral effect photodiode) for determining a first centroid of the light distribution and a DRPSD (e.g., a multiplexed array) for determining a second centroid of the light distribution within a reading window about the first centroid and within the light distribution. The second centroid represents the position of the light spot in the light distribution. Beraldin's multiple stage approach exploits the high resolution and speed offered by CRPSDs with the accuracy under variable lighting conditions offered by traditional DRPSDs.
The optical PSDs taught by the prior art can be used in many applications where remote, touch-free sensing and extremely high sensitivity are required. Some of these applications take advantage of a geometric leveraging effect to monitor mechanical devices. In accordance with this effect, the light beam is allowed to travel a large displacement across the face of the PSD in response to a small movement of the mechanical device being monitored. This approach allows the user to increase measurement sensitivity and decrease sensitivity to misalignments between the remote PSD and the mechanical device.
However, a high level of geometric leveraging creates a need for a large PSD. In addition, certain applications require that a large number of mechanical devices be monitored at the same time. Using a dedicated sensor for each device is extremely costly and not feasible or downright impossible due to physical constraints. It would be advantageous to resolve this problem by providing a single, large PSD having a sufficiently large bandwidth to sense a large number of multiplexed beams.
Unfortunately, the prior art technologies cannot be used for developing a large PSD with a sufficient bandwidth for monitoring a large number of parts. Specifically, in applications requiring a PSD as large as 50 mm×50 mm and a sampling rate of 25 MHz, even photodetectors built with CMOS are no longer fast enough. Thus, it would be a major improvement in the art to provide a single apparatus for monitoring the position of a large number of objects or mechanical parts while taking advantage of a high degree of geometric leveraging.